All-optical exclusive-OR (XOR) logic gate is a basic and crucial element for optical signal processing. We propose and demonstrate a novel high-speed all-optical XOR gate based on four-wave mixing (FWM) in a semiconductor optical amplifier, with optical return-to-zero differential phase-shift keying (RZ-DPSK) input signals. With this constant-envelope modulation format, the patterning effect-induced signal degradation in semiconductor optical amplifier (SOA) is alleviated. The proposed XOR gate can accommodate both two-input and three-input logic operations, which offer better flexibility for cascaded all-optical processing. Experimental demonstrations at 10- and 20-Gb/s verified the logic integrity of the proposed XOR gate. In addition, the XOR output tolerance against temporal misalignment among the data inputs is also experimentally investigated